There are only a few ways to tell. If you bred it's parents together and got him, you would know if he is a het. Though, it seems like you did not breed his parents to get him. The other ways to tell his breeding him with a female then breeding the babies back to him to see what the recessive gene was. Or, you could breed the babies to each other (well, once they get old enough). Though, some babies could carry the gene while others will not, so if you breed him with only one female who doesn't carry the gene, you won't get the morph he is a het for. You may just have to breed all the brothers and sisters together and see what you get. Remember, you can only inbreed snakes a few times before they start to get problems mentally and/or physically. You will have to breed him out to see, but you may never know if he is a het or not.